The present invention relates to current interrupting switching systems. More particularly, it deals with vacuum switches used in systems for interrupting the very large low voltage DC currents associated with electrolytic chemical cells, such as chlor-alkali cells. In such cells, several thousand amperes of current are continuously passed through a solution to effect separation of desired chemical constituents. Numerous cells are operated electrically in series at a low DC voltage which had been typically ten volts or less, but more recent cells operate at about fifty volts.
Periodic maintenance requirements dictate the need for low voltage, high current interrupting switching means for isolating a single cell from the remainder of the electrically series cells. A recent development has been to utilize vacuum switches, such as seen in U.S. Pat. No. 3,950,628, as the switching or current interrupting means with such cells. Other vacuum switches and the operating mechanism for such switches designed for use with electrochemical cells are set forth in copending applications Ser. No. 650,322 filed Jan. 19, 1976, and Ser. No. 650,406 filed Jan. 19, 1976, both of which applications are owned by the assignee of the present invention. A vacuum switch has at least one movable contact disposed within a hermetically sealed evacuated chamber. The switch or several parallel switches are shunt connected across the cell, and when maintenance is required on the cell, the contacts are closed to divert the current around the cell. The contacts of the switch are moved apart to the open switch position to place the cell back into the service.
Since the cells are typically operated at about ten volts or less, it is possible to separate the contacts and quickly extinguish the arc which forms between the contacts as they are separated. The contacts are typically copper or copper alloy, which exhibits a characteristic DC cathode drop potential in a vacuum, below which potential an arc cannot be maintained between separated contacts. For copper, this cathode drop potential is about twenty volts. The vacuum switch takes advantage of this cathode drop potential in extinguishing the arc.
With newer electrolytic cells the DC operating potential is about fifty volts. Since this voltage is above the cathode drop potential for most contact materials it is not possible to extinguish the arc with the vacuum switch, and thus vacuum switches have not been used with such higher DC voltage cells.
In the AC power transmission technology it is a common practice to use parallel vacuum interrupters, with series connected vacuum interrupters in one parallel path to boost the voltage withstand capability of the interrupter system. The series connected interrupters can withstand the rapid buildup of a high AC transient recovery voltage which is impressed across such interrupters shortly after the current zero interruption. In such AC systems, the voltage across the devices swings through zero facilitating interruption before the recovery voltage buildup.
In a DC vacuum switch system, there is no change in the voltage impressed across the system and extinguishment of the arc is achieved by separating the contacts and having the cathode drop potential for the vacuum switch exceed the DC line voltage for the system. No arc can be maintained under such a condition and there is arc extinguishment and interruption of the very high line current.